Halofluorobenzenes are useful intermediates for the manufacture of various dyes, agricultural pesticides, pharmaceutical and industrial compounds. For example, o-bromofluorobenzene may be converted to 3-fluorosalicylaldehyde for use in preparing oxygen absorbing solid state chelates such as "Fluomine" [cobalt bis(3-fluorosalicylaldehyde)ethylenediimine].
Conventional methods of preparing halofluoroaromatic compounds are based primarily on diazotization routes involving a number of steps. In U.S. Pat. No. 4,476,320, for example, halofluoroaromatic compounds were prepared by (a) diazotizing the corresponding haloaromatic amine compound to the diazonium salt, and (b) decomposing the salt to the desired product ##STR1## A similar scheme was employed in U.S. Pat. No. 3,950,444 and in Japanese Kokai Tokkyo Koho JP No. 59 67,232.
Alternatively, halofluorobenzenes have been prepared by the halogenation of fluorobenzenes, but the fluorobenzene starting materials are themselves usually prepared by the above-mentioned diazonium chemistry.
Of primarily laboratory interest, several techniques for the fluorination of halobenzenes have recently been disclosed. These procedures include fluorination with fluorine atoms (J.FluorineChem., 3, 397 (1973)), with acetyl hypofluorite (J.Org.Chem., 51, 1886 (1986)) and with AgF.sub.2 (J.Org.Chem., 45, 3597 (1980)).
Although highly fluorinate aromatic compounds can be prepared from perhalogenated aromatic compounds or perhalogenated aromatic compounds containing one or more electron-withdrawing substituents by the action of alkali metal fluorides, it was believed that this reaction was of preparative interest only for producing completely halogenated compounds and that reactions between incompletely halogenated aromatic compounds and KF were accompanied by numerous side reactions and poor yields. (See, for example, Yakobson et al. in Synthesis, 652, October 1976).
Shiley et al. in J.FluorineChem., 2, 19 (1972) disclose the fluorination of trichlorobenzenes with KF in dimethyl sulfone. Moderate yields (56 percent) of 1,3,5-trifluorobenzene were obtained, but only poor yields (less than 15 percent) of the 1,2,3- and the 1,2,4-trifluorobenzenes were achieved. In testing the stability of fluorobenzenes in halogen-exchange conditions milder than those required to produced the fluorobenzenes from their intermediates, Shiley et al. concluded that it would be difficult to find conditions for the halogen-exchange reaction that would be more conducive to better yields of the fluorobenzenes.